FORM OF THE PLANT-CELL. 



63 



bands consist exclusively of prosenchyma (figs. 54. a, 55. a), whilst the 

 wood between which they lie consists of very regular, not much ex- 

 tended, thin-walled, and porous parenchyma (figs. 54. c, 55. c). 



The opposite of this is the extremely light and porous wood of the 

 various species ofAvicennia, which consists almost entirely of very broad 

 porous vessels, whose interstices are filled with small porous parenchyma- 

 cells (fig. 56.). 



Lastly, the great mass of most wood consists of longitudinal, thick- 

 walled prosenchyma-cells, and, to a greater or less extent, of smaller 

 porous vessels (figs. 57, 58.). 



From the foregoing remarks and observations it will be seen that what 

 are called vessels are unessential modifications of the cellular tissue, and 

 care should be taken that no erroneous impression be conveyed by the 

 once generally received term of " vascular bundles." Such bundles may 



56 Transverse section of the wood of Avicennia. The wood consists entirely of very 

 broad porous vessels, together with very small thin-walled parenchymatous cells. 



67 Transverse section of the very heavy and thick wood of Mahonia nepalensis. The 

 entire mass consists of very thick-walled wood-cells (c), and broad porous vessels (i). 

 The cells of the medullary rays (a) are very thick-walled, and scarcely to be distin- 

 guished from the wood-cells. 



58 Longitudinal section of the same, a, b, c, correspond with fig. 55. d, the cut 

 cells of a large medullary ray. 



